Introduction to Current 3D Printing Technologies
3D printing has taken by the storm design companies all over the world. It allows designers to visualize the object in 3D. It is thus a prototyping process in which a real object is created from a 3D model. Since the raw material is added layer by layer to form the final object, 3D printing is also called as additive manufacturing or desktop fabrication.
You can create a 3D printing by scanning a set of 3D images, or drawing it using CAD software (like PTC Windchill - PLM). The digital 3D-model is usually saved in STL format and then sent to the printer.
| A word about the STL format
3D printers use a file format called STereoLitography (STL). It is used to store 3D object files for feeding to a 3D printer. STL files are an open file standard and are widely used in Computer Aided Manufacturing (CAM) and for rapid prototyping. The STL file format is good for printing out parts because it describes a part's surface geometry. |
3D printers are revolutionizing the way we manufacture things. As the technology progresses and the cost of printing reduce, 3D printers are finding more and more consumer acceptance.
At their core, 3D printers create objects by controlling the placement and adhesion of successive layers of a suitable 3D printing material in 3D space. This is the reason why 3D printers are also called as additive layer manufacturing printers.
There are various technologies used in 3D printers. They are broadly based on the following technologies:
i. Material Extrusion based 3D printers – these form object layers by outputting a semi-liquid material from a print head nozzle.
ii. Photo Polymer – they work by selectively solidifying a liquid resin called as a photpolymer that hardens when exposed to a laser or any other light source.
iii. Selective Deposition Lamination - successive layers of cut paper, metal or plastic are stuck together to build up a solid object. They can also be used to print in 3D colour.
iv. Binding 3D printers – such printers create objects layers by selectively amalgamating the granules of a very fine powder. Such ‘granular materials binding’ can be achieved by jetting an adhesive onto successive powder layers, or by fusing powder granules together using a laser or other heat source.
The ISO/ASTM 52900 standards introduced in December 2015 identify seven 3D printing manufacturing technologies. As per ISO norms, these standards will next be reviewed in 2020. All of the current 3D printing technologies are broadly based on the above classification.
Just like everything else in real life, actual 3D printers are complex to define. There are various manufacturers that use their own patented material and technologies, and call them by different names. Below, we are detailing the most common types of 3D printers, based on the kind of processes and material used:
1. Material Extrusion
These printers are by far the most common 3D printers. They work by shaping up an object layer by layer from a semi-liquid material. This material is squeezed out of its container – just like toothpaste – from a computer controlled nozzle. In the printing process, the nozzle continuously moves around, extruding the chosen material in a precise manner and building the object layer by layer. Once it is deposited, it cools instantly. There are many materials that can be used for material extraction 3D printers, including ceramics and metals. However, the most common of choice is a class of plastics called thermoplastics (plastics that liquefy on heating and solidify on cooling).
Stratasys, a pioneer and a leader in the manufacturer in 3D printers coined and trademarked the term FDM (Fused Deposition Modelling) for their process.
3D printers based on FDM technology / material extrusion are ideal for prototypes that do not require very high precision.
2. Vat Photopolymerization
As defined above, 3D printers that use this technology are based on the photo polymeric properties of resins to build an object. Resins are of two kinds – natural and synthetic. They consist of a non-crystalline or viscous liquid substance. The resins used for 3D printing are thermosetting resins that solidify on exposure to heat / laser. These printers solidify successive object layers on the surface or base of a vat of liquid photopolymers when they are exposed to lasers or light.
These 3D printers can be further subcategorized as:
2a. Stereolithography (SLA) Printers
Stereolithography (SLA) – Liquid Based 3D Printing
These were one of the first commercial 3D printers. They use a computerized laser beam to build a 3D object layer by layer within a vat (or tank) of liquid photopolymer.
A stereolithographic printer has four main parts – the raw material vat containing photopolymer, a perforated platform that is gradually lowered into the vat, a computer connected to the entire assembly to control the movement of the platform and a UV laser that heats and solidifies the photopolymer.
Objects are formed on the perforated platform that is positioned just under the surface of a vat containing photopolymer raw material. The object is created step by step using a laser beam that traces out the shape of the object.
2b. Continuous Liquid Interface Production (CLIP)
A relatively new entrant in the field of 3D printing technology, the Continuous Liquid Interface Production (CLIP) uses different thermoplastic engineering technologies to produce 3D objects that have great finishes and resolution. These printers use UV curable resin as the raw ingredient, an oxygen permeable slot with a dead zone, projector and a build platform.
The CLIP process works by creating a persistent liquid interface by positioning an oxygen-permeable window below the ultraviolet image projection plane. This 'dead zone' inhibits photopolymerization by balancing oxygen and the UV laser to cure the resin.
By avoiding the steps to cure, replenish, and reposition the platform for each additive cycle, 3D printers based on CLIP technology work faster.
2c. Digital Light Processing (DLP)
These 3D printers work on the same principles as stereolithography. The main difference is that instead of using special UV lasers, these printers use traditional light sources to solidify the photopolymer material and create a 3D object. The image formed by each layer of the digital screen projector is composed of square pixels, resulting in a layer formed from small rectangular bricks called voxels. Printers based on DLP technology work faster than the traditional 3D printers for objects where an entire layer can be carved at the same time.
In general, 3D printers based on the SLA printing technology are useful for printing 3D jewellery, medical applications and dental applications.
3. Material Jetting Printers
These printers work by jetting state of the art photopolymer materials in ultra-thin layers onto a build tray layer by layer until the part is completed.
Each photopolymer layer is cured by UV light immediately after it is jetted, producing fully cured models that can be handled and used immediately, without post-curing. The gel-like support material, which is specially designed to support complicated geometries, is easily removed by hand and water jetting.
This technology helps in printing rigid parts, transparent parts as well as flexible parts that are required for prototyping applications. This is the only technology which can print multi-materials and multi-colours in a single build. Stratasys is a major player in this field. The company calls their 3D printers that use this technology as PolyJet printers (Polymer Jetting). Being a pioneer, Stratasys has a significant market share for Polyjet printers in India and abroad.
4. Powder Bed Fusion Printers
Another broad category of 3D printing technology is Power Bed Fusion that uses either a laser or electron beam to melt and fuse material powder together. There are many 3D printing techniques that are based on this technology. They include:
4a. Selective laser sintering (SLS)
4b. Direct metal laser sintering (DMLS)
4c. Selective laser melting (SLM)
4d. Selective heat sintering (SHS)
4e. Electron beam melting (EBM)
Essentially, the variations arise due to a combination of lasers, electronic beams and the build material used.
Irrespective of the process, here are the basic steps in any Powder Bed Fusion printing:
1. The raw material is spread on the build platform in layers.
2. Depending on the 3D techniques, a laser or an electronic beam then fuses this layer into desired shape.
3. A new layer of raw material is spread on this layer.
4. More layers or cross sections are fused and added
This process is repeated until the desired object is created.
A few advantages of the powder bed fusion technology include:
prototypes are inexpensive to manufacture
the powder acts as an integrated support structure
wide range of raw material available
5. Binder Jetting
This is a 3D printing technology in which a suitable liquid binding agent is selectively deposited to join powder particles. In this additive prototyping process, the powder between each layer is cured a bit for solidification. These types of 3D printers use the print-head to drop the binder onto the raw material precisely as per the 3D model. When the printing process is complete, the build box is removed from the printer. The prototype is then placed in then usually put in an oven to solidify.
3D printers based on the binder jet principle do not require any support structures, as the built platform lies in the bed of un-bonded powder. One of the main advantages of binder jetting is that it works with almost any material that is available in powder form.
6. Direct Energy Deposition
Direct Energy Deposition 3D printers cover a range of different technologies. They refer to a 3D printing process in which the raw material is directed into a spatial location where the energy input and the desired deposition site are co-located. There are many variations in the actual process used depending upon the manufacturer - from Electron Beam Direct Manufacturing (EBDM) / Electron Beam Freeform Fabrication (EBF3) to Laser Powder Forming (LPF). Like other 3D printing technologies, EBDM and EBF3 are basically the same processes developed by different companies.
Working of Direct Energy Deposition 3D Printers
A typical 3D printer based on Direct Energy Deposition technology consists of a nozzle (or head) that is mounted on a multi axis arm. It has multiple jets that supply the raw material for building the prototype. The nozzle can move spatially along the X, Y and Z directions, and is usually mounted on a CNC machine or on an industrial robot. The angle of deposition of the raw material is adjustable. The released build material is added to a shielding gas that is released out of the jets to create an inert work zone. The build material is then heated with lasers or an electronic beam. Once it melts, the nozzle moves and gives it the desired shape. The material then solidifies, creating a weld track.
One intrinsic limitation of the direct energy deposition 3D Printers are the surface finish and dimensional tolerances. Although they are just getting introduced into mainstay 3D printing, their primary use is in repairing and adding material to existing components.
7. Laminated Object Manufacturing
Laminated Object Manufacturing is yet another 3D printing technology. It uses lasers to form and fuse layers of plastic or paper into laminations and bonding them into a stack. This is done using heat and pressure. The laminates are then cut into the desired shape with a computer controlled laser beam. The 3D printers that are based on Laminated Object Manufacturing use a continuous sheet of raw material that can vary from paper to metal. The build material is often coated with an adhesive to laminate it properly. After the printing is complete, the excess material is simply cut away.
An inherent advantage of creating models using this technique is that there is virtually no internal stress, especially if the temperature is reduced gradually. The disadvantage is that these 3D printers cannot create hollow objects or objects with complex geometry.